Plasma etching is one process used in the definition of the structure of a silicon integrated circuit. One example involves the etching of via holes through a dielectric layer to form a vertical metallic interconnect structure, which in some advanced designs may simultaneously form the horizontal interconnect structure. The dielectric layer is conventionally formed of a material based on silicon dioxide, also called oxide. More advanced dielectrics have included fluorine or other dopants to reduce the dielectric constant. Yet other dielectric compositions may be used. The long established photolithographic process deposits a generally planar layer of photoresist material onto the unpatterned oxide with perhaps an anti-reflective coating (ARC) therebetween. The photoresist is optically patterned according to a desired patterned and then developed to remove the unexposed photoresist in positive lithography or exposed photoresist in negative lithography. The patterned photoresist then serves as a mask for a further step of etching the exposed oxide and intermediate ARC if present. Dielectric etch processes have been developed which provide a reasonable etch selectivity between the oxide and photoresist.
The advance of integrated circuit technology has depended in large part on the continuing shrinkage of the horizontal features such as the via holes through the oxide layer. Via widths are now decreasing to below 100 nm. However, the oxide thickness has held steady at around 1 μm and there are many structures in which oxide thicknesses of 3 μm or more are desired. Such high aspect ratios of the holes to be etched in the oxide layer have presented several problems between the photolithography and the etching. To maintain depth of field in the optical patterning, the thickness of the photoresist should not be much greater than the size of the feature being defined in the oxide layer, e.g., 100 nm in the above example. As a result, the etch selectivity, that is, the ratio of the oxide etch rate to the photoresist etch rate must be 10 or greater if the mask is to remain until the via hole has been etched to its bottom. However, photoresists are typically based on soft organic materials. Obtaining such high selectivity of photoresist has been difficult to achieve while simultaneously achieving other requirements such as vertical profiles in the narrow via holes.
It is desired to transition the lithography from 248 nm radiation for exposing the photoresist from a KrF layer to 193 nm radiation from an ArF laser. However, the 193 nm radiation presents problems. The photoresist sensitive to the shorter wavelengths is generally softer and the maximum thickness of the photoresist is generally reduced to less than 400 nm to accommodate the shallower depth of field at the shorter wavelength.